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U.S. GEOLOGICAL SURVEY
Data Series 280

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Major and Trace Element Chemistry

Most of the borehole samples (34 of 48) were readily soluble losing 90.9 to 100 percent of their original sample weight during the 0.2 molar nitric acid treatment (table 5). The remaining 14 samples lost between 4.1 and 86.8 percent of the original sample weight. The greater amounts of residue in these samples are assumed to consist mostly of silicate material. Most samples were readily identifiable as limestone or dolomite, based on their CaO and MgO contents (diamonds on fig. 4). Assuming that all of the dissolved CaO and MgO is derived from the carbonate fraction, calcite (CaCO₃) mole percentages vary from 93.2 to 99.6 percent in limestone samples and 48.0 to 53.6 percent in most dolomite samples. A small number of samples have intermediate MgO contents, resulting in calcite mole percentages between 55.6 and 74.2 percent. These values exceed the amount of calcite that can substitute into the dolomite crystal structure (Gaines and others, 1997, p. 450). The resulting linear trend between calcite and dolomite stoichiometric end members indicates that these samples represent mixtures of the two phases. Samples with intermediate compositions include core, cuttings, and a discrete sample of sparry vein carbonate [Army#1(982) B]. This observation indicates that excess CaCO₃ contents are intrinsic to some dolomite samples and are not the result of down-hole mixing of cutting fragments.

In contrast, three leachate samples with 0.95 to 11.5 weight percent MgO concentrations plot off the calcite-dolomite mixing trend and have elevated SiO₂ concentrations compared to other carbonate leachates (solid squares, fig. 4). These leachates —ER16-1 (3540), UE10j (2051), and UE16d (1550)— also had the lowest percent-sample-dissolved values (4.1 to 24.5 percent) and contained noticeable clay or volcanic constituents. These observations indicate that chemical compositions of these leachates are not derived exclusively from carbonate components and likely include ions leached from silicate minerals.

Measurable amounts of SiO₂ and Al₂O₃ in leachates indicates that small amounts of clay minerals may be present in many of the carbonate samples. The positive correlation between SiO₂ and Al₂O₃ concentrations with a SiO₂:Al₂O₃ weight ratio of 2:1 (fig. 5A) could be the result of variable amounts of clay minerals mixed in with the marine carbonate rocks. Systematic differences in SiO₂ concentrations between dolomite and limestone samples are not apparent (fig. 5B).

Concentrations of Sr vary widely, but systematically, in the carbonate fractions of the 2006 borehole samples (fig. 6). Limestone samples have Sr concentrations ranging from 97.5 to 1,680 micrograms per gram (µg/g) with a median value of 384.5 µg/g (N=16) whereas dolomite samples have Sr concentrations ranging from 16.9 to 146 µg/g with a median value of 46.5 µg/g (N=21). Carbonate samples with CaCO₃ mole percentages between 55 and 74 percent have intermediate Sr concentrations with a median value of 140 µg/g (N=11).

Concentrations of Rb in leachates of most samples are less than the minimum reporting limits of around 1 µg/g (table 5). Rb concentrations generally increase with increasing SiO₂ (fig. 7A) indicating that silicate minerals (most likely clays) are a repository of Rb. Dolomite and limestone samples have similar Rb concentrations, but because of their lower Sr concentrations, dolomite samples tend to have higher Rb/Sr ratios (fig. 7B). Most samples have Rb/Sr ratios less than about 0.05. For a Cambrian marine carbonate sample with a Rb/Sr ratio of 0.05 and an initial ⁸⁷Sr/⁸⁶Sr value of 0.70900 (δ⁸⁷Sr = ‑0.28), the decay of ⁸⁷Rb during the last 500 m.y. will result in a present-day ⁸⁷Sr/⁸⁶Sr value of 0.71003 (δ⁸⁷Sr = 1.17), an increase of about one in the third decimal place. If the same rock had a Rb/Sr ratio of 0.015 (the upper limit for most dolomite and limestone samples, fig. 7B), the present-day ⁸⁷Sr/⁸⁶Sr ratio would only evolve to 0.70931 (δ⁸⁷Sr = 0.15). Only four clay-rich samples have Rb/Sr ratios higher than 0.056 (fig. 7B).

Additional elements reported in table 5 include MnO, Th, and U. Concentrations of MnO are statistically uncorrelated with mole percent CaCO₃ contents (linear correlation coefficient, r², of 0.035) although the highest MnO concentrations are present in limestones. Samples with the lowest SiO₂ concentrations also tend to have the lowest MnO and Th concentrations indicating the potential influence of clay minerals and the low solubility of these constituents in marine environments. In contrast, U concentrations are uncorrelated statistically with SiO₂ and are present in similar concentration ranges in dolomite and limestone samples.

Because CaO and MgO concentrations are not available from XRF analyses, quantification of calcite mole percentages in outcrop samples was not possible. Bulk-rock Sr concentrations range from 16 to 2,537 µg/g with most samples having between about 30 and 500 µg/g (table 6). Samples with Sr concentrations greater than 500 µg/g typically increase along with Zr concentrations (fig. 8A). Because Zr is a highly insoluble element in marine environments (mean Zr concentration in North Pacific Ocean water is 0.000015 µg/g; McKelvey and Orians, 1993) compared to Sr (mean seawater Sr concentration of 7.74 µg/g; Faure and Mensing, 2005, p. 437), Sr concentrations greater than 500 µg/g in outcrop samples likely are attributable to silicate constituents included in the rock. Concentrations of Rb typically cluster at values less than 5 µg/g and are not correlated with Sr concentrations (r² value of 0.039 in fig. 8B). However, the crude correlation between Rb and Zr concentration (r² value of 0.43 in fig. 8C) implies that both constituents are derived from terrigenous silicate components. Therefore, outcrop samples with the lowest Zr concentrations best represent the carbonate fraction of these bulk-rock analyses. A frequency distribution plot of Sr concentrations in samples having Zr concentrations less than 20 µg/g (fig. 8D) indicates a narrow frequency peak between 25 and 75 µg/g (mostly classified as dolomites in field descriptions) and a broader frequency peak between 100 and 400 µg/g (mostly classified as limestones).

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